The present invention relates to a rotary development device which has a rotating member constituted by a rotary frame and a plurality of development units detachably mounted on the rotating member along a cylindrical outer circumference thereof and in which the development units are selectively rotated to a developing position opposite to an image bearing member to develop a latent image on the image bearing member, the invention also relating to an image forming apparatus.
Also, the present invention relates to an image forming apparatus having a configuration in which a plurality of development units are sequentially rotated to a developing position as a result of developing color switching operations of a rotary development device to form an image by developing a latent image formed on an image bearing member and transferring it onto a medium in a transfer position.
Image forming apparatus such as copying machines, printers, facsimile machines utilizing electrophotography include a type of apparatus in which electrostatic latent images in a plurality of colors are sequentially formed on a photosensitive member. The electrostatic latent images are sequentially developed into toner images in the respective colors with a rotary development device; the toner images in the respective colors are thereafter transferred onto an intermediate transfer medium in an overlapping relationship. A resultant full-color toner image transferred onto the intermediate transfer medium is transferred to a transfer member and is then fixed thereon.
In the above-described rotary development device, the rotating member is provided opposite to a photosensitive member; the plurality of development units (development units for yellow, cyan, magenta, and black) are mounted in the rotating member; the rotating member is rotated to put any of the plurality of development units in contact with a developing position of the photosensitive member selectively; and a developing bias is applied while rotating a development roller in the development unit to develop an electrostatic latent image into a toner image.
The color image forming apparatus having a rotary development device include a type of apparatus in which a development unit is urged toward a photosensitive member to put a roller, which is provided coaxially with a development roller, in abutment with the photosensitive member to position the photosensitive member in order to maintain a proper gap between the photosensitive member and the development roller. In this type of apparatus, variation of development units in the form of cartridges mounted on a rotary development device affects engagement between a drive input gear and a drive output gear, and the state of engagement between the drive input gear and the drive output gear varies for each cartridge or development unit of the respective color. From this point of view, proposals have been made on apparatus having a configuration in which an idler gear is provided in a position associated with each of development units of a rotary development device and in which a train of gears of the development unit is engaged with the idler gear, and the idler gear is engaged with a drive output gear in a developing position, the configuration allowing a development roller to be urged toward a photosensitive member with stability when a drive for development is transmitted and making it possible to maintain a gap between the photosensitive member and the development roller easily (see JP-A-2003-5484).
In the above related apparatus, a development unit is urged by the elastic member from behind and, in addition, a transmission reaction force generated by a drive for development is exerted in the direction of urging the development unit against the photosensitive member to rotate the development unit about the fulcrum and to urge it against the photosensitive member. Further, the fulcrum is inserted in the positioning hole of the development cartridge. Vibration coming from the driving source can be absorbed at the train of gears transmitting the drive to the development unit by urging the development unit against the photosensitive member utilizing such an elastic member or a transmission reaction force or by designing the material of the gear appropriately. In the fitting portion of the fulcrum, there is considerable looseness of fitting between a pin at the fulcrum and a hole to be fitted with the same in order to accommodate variation of the accuracy of the components and to allow the development cartridge to be smoothly attached and detached. Since vibration from the driving source primarily generated at the time of driving for development acts on such as loose fit, the development unit itself will have troubles such as an undesirable movement or resonance, which will results in the problem of image defects such as banding.
Further, an image forming apparatus employing the rotary developing method includes an image bearing member on which a latent image is formed, a plurality of development units which have a development roller and which convey a developing agent to develop a latent image, and a rotary development device which supports the plurality of development units along an outer circumference thereof and rotates them to a developing position opposite to the image bearing member. In the mage forming apparatus, a development unit performs a developing operation with the rotary development device positioned so that the axis of rotation of the rotary development unit, the axis of rotation of a development roller, and the axis of rotation of a photosensitive member are aligned on a straight line in the developing position (for example, see JP-B-2649033 and JP-A-2003-5484).
However, when a development unit performs a developing operation in the developing position in the related apparatus, some problems arise because the rotary development device is positioned so that the axis of rotation of the rotary development device, the axis of rotation of the development roller, and the axis of rotation of the photosensitive member are aligned on a straight line.
For example, when the developing operation is finished and the rotary development device is rotated by a developing color switching operation to rotate the development unit for the next color to the developing position, the urging force which has urged the development unit in the developing operation so far against the photosensitive member begins to decrease from a maximum value as the movement begins and becomes zero when the development roller completely separates from the photosensitive member. There are various methods for determining the distance between a development roller and a photosensitive member in a developing position. In an example of a non-contact developing method, a roller having a diameter slightly larger than that of a development roller is disposed coaxially with the development roller and the roller is put in abutment with a photosensitive member to determine the gap between the development roller and the photosensitive member. In a case that the above description of reduction of an urging force is applied to this configuration, there is zero urging force when the roller separates from the surface of the photosensitive member some time after the rotary development device rotates out of the developing position. In JP-A-2003-5484, the roller is described as a circular member at an edge of a development roller 122 as shown in FIGS. 2 and 3.
A change in the urging force that urges a development unit against the photosensitive member results in a change in a load for driving the photosensitive member. This causes an irregularity in the rotation of the photosensitive member and causes an image defect such as an irregularity in an image. There is some looseness when the photosensitive member is mounted in a main body of an apparatus. A certain degree of looseness is required for a structure to which the member is fit-mounted. When the development unit is urged against the photosensitive member, the photosensitive member moves in a leaving direction of leaving from the development roller on a straight line connecting the axis of rotation of the rotary development device, the axis of rotation of the development roller, and the axis of rotation of the photosensitive member, the movement staying within the range of such looseness. When the rotary development device starts rotating and the urging force decreases consequently just as described above in relation to the above-mentioned problem, the photosensitive member can move in the direction opposite to the leaving direction within the range of the looseness. Such changes in the position of the photosensitive member result in the problem of distortion of an image between a primary transfer roller and the photosensitive member (transfer deviation).